During the coming year we plan to take the following research approaches. First, to determine if the Fv-2 gene acts early or late in the growth cycle of SFFV, we have begun a collaborative study to compare the amount of SFFV-specific viral RNA synthesized in the spleens of partially congenic Fv-2r and Fv-2s mice. Second, to determine if the Fv-2 gene acts by controlling the fraction of primitive erythroid cells that enter the cell cycle, we will apply new assays for target cells of SFFV and for primitive erythroid cells to characterize factors at both the host and cellular levels that promote or prevent Fv-2r-mediated resistance. Third, to determine if the lethality observed in Fv-2r/Fv-2r homozygotes after 14 backcrosses to the DBA/2 strain was a consistent or coincidental event, we plan to repeat the backcross experiments and test for lethality at each generation by inter-crossing Fv-2s/Fv-2r heterozygotes. Finally, genetic studies will be carried out to confirm the apparent existence in the C57BL mouse of a gene which protects these mice from the lethality of the homozygous Fv-2r genotype.